Pure and Binary Gas Adsorption Equilibrium for CO2âN2 on Oxygen

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Pure and binary gas adsorption equilibrium for CO2-N2 on oxygen enriched nanostructured carbon adsorbents Chitrakshi Goel, Deepak Tiwari, Haripada Bhunia, and Pramod K. Bajpai Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02671 • Publication Date (Web): 19 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017

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Pure and binary gas adsorption equilibrium for CO2-N2 on oxygen enriched nanostructured carbon adsorbents Chitrakshi Goel, Deepak Tiwari, Haripada Bhunia,* Pramod K. Bajpai [email protected], [email protected], [email protected], [email protected] Department of Chemical Engineering, Thapar University, Patiala-147004, Punjab, India

*Correspondence to: Haripada Bhunia Email address: [email protected] Phone: +91-175-2393439; fax: +91-175-2393005


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ABSTRACT Pure component (CO2 and N2) adsorption isotherms of oxygen enriched nanostructured carbon (RF-700) were evaluated using a static volumetric analyzer at four different adsorption temperatures ranging from 30 to 100 °C. Langmuir, Sips, and dual-site Langmuir (DSL) models were used to correlate pure component adsorption isotherms and it was found that Sips and DSL isotherm model fitted well with the experimental data, indicating heterogeneous nature of the adsorbent surface. Fixed-bed column was used to obtain dynamic breakthrough data for binary system CO2-N2 at different adsorption temperatures (30-100 °C) and CO2 feed concentrations (512.5% by volume). Extended Sips, Extended DSL and IAST (ideal adsorbed solution theory) models using pure component adsorption isotherm data, were used for the prediction of adsorption of binary system (CO2-N2). Predicted equilibria data was compared with experimental breakthrough curve data and it was found that extended forms of the isotherm models (Sips and DSL) under-predicted CO2 adsorption equilibria because of difference in adsorptive strengths of CO2 and N2 molecules. Ideal adsorbed solution theory failed to describe the mixed-gas adsorption equilibria. Asymmetric x-y diagrams showed positive deviation from Raoult’s law. Feasibility of the adsorption process was suggested by negative value of molar Gibbs free energy change. Formation of more ordered configuration of CO2 molecules on adsorbent surface was seen as higher heat of adsorption was exhibited for CO2 as compared to N2. Keywords: Oxygen-enriched carbon; Pure component adsorption; Binary gas adsorption; Carbon dioxide; IAST


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1. INTRODUCTION Increase in concentration of anthropogenic carbon dioxide (CO2) in the environment is leading to global warming. This increase is mainly due to combustion of fossil fuels such as coal, petroleum and natural gas which are the major sources of energy. CO2 concentration has reached a value of around 403.6 ppm1, 2 as per the current scenario. This is further increasing the global temperature and sea levels which are making a bad impact on the climate and the marine life. Therefore, reducing the concentration of CO2 is the current need of our environment. Carbon capture and sequestration (CCS) strategy has a potential to inhibit the increase of CO2 concentration.3-7 CCS includes capture of carbon dioxide by various methods like absorption, adsorption, membrane separation and cryogenic distillation.8,

9

Large scale CO2 capture in

industries is carried by amine-based absorption but it has intrinsic limitations like low efficiency, equipment corrosion, high energy requirement and solvent loss.10 As a substitute, adsorption by solid adsorbents has gained much attention because of reducing energy penalty, high adsorption capacity, selectivity and fast kinetics coupled with good thermal and mechanical stability.11 Among all potential adsorbents such as zeolites,12 activated carbons,13, frameworks,

15, 16

17, 18

amine supported mesoporous materials,

14

metal-organic

etc., carbon based adsorbents have

been extensively used because of their better properties as compared to others like welldeveloped porous structures, high adsorption capacities, fast kinetics, easy regenerability and high thermal and chemical stability.11 Their preparation can be carried out from a wide range of low cost sources by using various techniques like sol-gel, carbonization of carbon containing precursor and nanocasting technique.19 Moreover, their adsorption capacity and selectivity can be enhanced by incorporation of heteroatoms like nitrogen, oxygen etc.19 in the carbon matrix which enhance the affinity of the adsorbent towards CO2.2, 20 Thus, for selectively capturing CO2, carbon adsorbents obtained from high oxygen content resorcinol formaldehyde resin and template mesoporous silica have been used.21 Understanding of pure component adsorption isotherms study is very crucial, in order to understand the capture potential of the adsorbents. Also, for designing an economically feasible adsorption system for CO2 capture, knowledge of mixed gas adsorption equilibria is important as it plays an important role. Therefore, using adsorption isotherm models and experimental data of pure component adsorption equilibria, binary gas adsorption equilibria can be predicted.22 Predicting mixed-gas adsorption equilibria using this approach is preferred because experimental


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determination is highly time consuming. But, it is necessary to check the accuracy of predicting models by using a set of experimental data for the same. Until now, very few studies have been performed for predicting mixed-gas adsorption equilibria on carbon based adsorbents from pure component adsorption. Moreover, the few which have been performed are for pre-combustion capture conditions. CO2, N2, H2 ternary system on a commercial activated carbon adsorbent was studied to evaluate the breakthrough time and equilibrium CO2 adsorption capacity for precombustion CO2 capture.23 In other work, Schell et al.24 presented pure component equilibria of CO2/H2/N2 and binary equilibria of CO2/H2 and CO2/N2 obtained gravimetrically on commercial activated carbon. In the present work, we predicted binary gas adsorption data for CO2 and N2 on oxygen enriched nanostructured carbon and compared with the breakthrough data. In this study, we have worked on post-combustion capture conditions and to the best of our knowledge, this is the first study to be carried out on this material. Pure component (CO2 and N2) adsorption equilibria was determined volumetrically at four different temperatures (30–100 °C). Study was carried out using three different pure component adsorption isotherms models namely, Langmuir, Sips and dual-site Langmuir (DSL) models and they were fitted with the experimental data. Also, binary gas adsorption equilibria data, obtained from fixed-bed adsorption system, were compared with predicted data obtained from extended forms of pure component adsorption isotherms models. The present study is much more effective because it was performed in dynamic fixed bed system in which we have evaluated CO2 uptake from 30 °C to 100 °C along with regenerability and selectivity which indicates novelty of the present work.

2. MATERIALS AND METHODS 2.1. Materials Oxygen enriched nanostructured carbon adsorbent (RF-700), prepared from resorcinolformaldehyde resin (precursor) and template mesoporous silica using nanocasting technique, has been used. Details of the synthesis process, characterization, dynamic performance evaluation studies, etc. were reported in our previous work.21 The oxygen content of our sample was 32.23%. Pure and binary gas adsorption studies were performed by using pure N2 (99.995%) and CO2 (99.999%) gases, procured from M/s Sigma Gases and Services (India). Non-adsorptive gas,


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helium (He) of high purity (99.999%) from M/s Sigma Gases and Services (India) was used for outgassing the sample. 2.2. Methods 2.2.1 Equilibrium sorption measurements Equilibrium adsorption-desorption experiments of pure gases (CO2 and N2) on nanostructured carbon adsorbent were performed using a Micrometrics ASAP 2010, volumetric analyzer at four different adsorption temperatures (30, 50, 75 and 100 °C) and pressure ranging from 0 to 1 atm using high purity CO2 and N2 gases. Before each adsorption experiments, carbon sample was degassed for 12 h under vacuum at 200 °C. Adsorption temperature was achieved by using a Dewar, having a circulating jacket and was connected to a thermostatic bath. 2.2.2 Binary component adsorption measurement Fixed-bed adsorption (Fig. 1) study set up coupled with an online gas chromatograph was used to evaluate the dynamic CO2 adsorption-desorption performance of the prepared carbon adsorbent. Adsorption setup detailed description and procedure was reported in the previous study.21 In a typical adsorption/desorption experiments, about 2 g of dry adsorbent along with dry inert glass beads is packed into the fixed bed reactor (internal diameter of 0.939 cm and height of 30 cm). Pretreatment of adsorbent was carried out for 2 h at 200 ˚C in nitrogen atmosphere (50 ml min-1) for the removal of moisture and other adsorbed gases. Hereafter, adsorption was carried out by lowering down the temperature to the desired adsorption temperatures (30, 50, 75 and 100 °C). Different gas mixtures of CO2 (5 to 12.5%) and N2 at a total flow rate of 80 ml min1

were used for adsorption study. The CO2 concentration at the bed exit was monitored as a

function of time until the exit CO2 concentration became equal to inlet CO2 concentration. After performing adsorption study, desorption was carried out by increasing the temperature to 200 °C under pure N2 flow (50 ml min-1).


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I.D. = 0.939 cm Height = 30 cm

Gas chromatograph: 7890A series, Agilent Technologies, USA Adsorption study setup: fabricated by Chemito Technologies Pvt. Ltd., India

Fig. 1 Schematic diagram of the CO2 adsorption/desorption setup


Fig.

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3 Result and discussion 3.1 Pure component isotherms Fig. 2 displays pure component CO2 and N2 adsorption isotherms on prepared oxygen enriched carbon adsorbent as a function of pressure at different adsorption temperatures (30100 °C). Experimental data are represented by symbols while isotherm models are represented by lines. The corresponding model parameters along with SSE (sum of the squared relative errors) values are reported in Tables 1, 2 and 3. Langmuir adsorption isotherm model exhibits correct asymptotic behavior as it approaches Henry’s law in the low pressure range and is thermodynamically sound. However, Langmuir equations under predicted CO2 uptake during initial adsorption phase showing higher SSE%. On the other hand, Sips and DSL equations were able to capture CO2 adsorption behavior properly at all adsorption temperatures and showed maximum SSE% of 4.4% and 5.2%, respectively. This indicated energetically heterogeneous adsorbent surface. Similar results of conformity of these two models with CO2 adsorption on carbon materials were shown by several research groups.25, 26 In case of pure N2 adsorption on RF-700, all the models used in this study can illustrate the experimental data with small errors (< 5%). This is because of linear shape of N2 isotherm that can be fitted well with all aforementioned isotherm models.26


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Fig. 2 Experimental adsorption isotherms of (a) CO2 and (b) N2 on RF-700 at different temperatures and their corresponding isotherm model fits


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Table 1. Langmuir isotherm model parameters for pure component adsorption on RF-700 at different temperatures Temperature (°C) Component

CO2

N2

Parameter 30

50

75

100

qm (mmol g-1)

2.44

2.16

1.84

1.71

b (atm-1)

2.22

1.46

0.85

0.49

SSE (%)

13.01

11.30

9.45

9.02

bo (atm-1)

0.0008

B (kJ mol-1)

-20.23

qm (mmol g-1)

1.49

1.23

0.98

0.79

b (atm-1)

0.14

0.12

0.09

0.08

SSE (%)

1.63

4.11

3.71

3.48

bo (atm-1)

0.005

B (kJ mol-1)

-8.52


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Table 2. Sips isotherm model parameters for pure component adsorption on RF-700 at different temperatures Temperature (°C) Component

Parameter 30

50

75

100

qm (mmol g-1)

3.47

3.15

2.61

2.13

b (atm-c)

0.98

0.71

0.49

0.36

c

1.27

1.21

1.13

1.06

SSE (%)

4.11

3.18

0.56

4.39

CO2 bo (atm-1)

0.005

B (kJ mol-1)

-13.47

qm (mmol g-1)

1.37

1.24

0.82

0.69

b (atm-c)

0.16

0.12

0.11

0.09

c

0.99

0.98

0.96

0.96

SSE (%)

1.08

0.89

2.49

4.54

N2 bo (atm-1)

0.0098

B (kJ mol-1)

-6.91


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Table 3. Dual-site Langmuir isotherm model parameters for pure component adsorption on RF-700 at different temperatures Temperature (°C) Component

CO2

N2

Parameter 30

50

75

100

q1,m (mmol g-1)

2.62

2.50

2.19

2.05

q2,m (mmol g-1)

0.45

0.26

0.09

0.04

b1 (atm-1)

0.99

0.75

0.55

0.35

b2 (atm-1)

12.36

9.77

8.45

6.030

SSE (%)

3.22

2.70

3.36

5.15

bo,1 (atm-1)

0.004

E1 (kJ mol-1)

-13.86

q1,m (mmol g-1)

0.75

0.62

0.49

0.39

q2,m (mmol g-1)

0.75

0.62

0.49

0.39

b1 (atm-1)

0.14

0.12

0.09

0.07

b2 (atm-1)

0.14

0.12

0.09

0.07

SSE (%)

1.63

4.11

4.71

3.48

bo,1 (atm-1)

0.005

E1 (kJ mol-1)

-8.51

Exothermic nature of the adsorption process was signified by decrease in affinity parameter (b) of these isotherm models with increase in adsorption temperature. DSL equation parameters b1 and b2 are equal for N2 adsorption on this carbon. Therefore, amount of adsorbate adsorbed will be same indicating no energetic site-matching issue and CO2 (component 1) will see site 1 as high free energy site and site 2 as low free energy site. So, there will be only one value of adsorbed amount for an adsorbate at a given temperature and gas composition conditions. Sips isotherm model parameter value c shows deviation in value from unity indicating heterogeneity of the adsorption process. Also, higher affinity of CO2


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towards adsorbent surface as compared to N2 observed from higher values of b in Tables 1, 2 and 3. Moreover, higher qm values of CO2 adsorption than N2 were obtained by all the three isotherm models.

3.2 Binary component isotherms Sips and DSL isotherm models were used for binary component adsorption as these models properly explain the pure component adsorption behavior. Also, IAST (ideal adsorbed solution theory) was only applied for the isotherms of these two pure component models only. Breakthrough curve of binary mixture (CO2 and N2) at 12.5% CO2 concentration (by volume) and at different temperatures (30 °C and 50 °C) is shown in Fig. 3. Hump in C/CO value, i.e. >1 is seen from Fig. 3, which indicates that nitrogen comes out first and gets less adsorbed as compared to CO2. Maximum CO2 adsorption capacity of 0.76±0.05 mmol g-1 at 30 °C under 12.5% CO2 concentration is obtained for this sample. The obtained capacity is much higher than CO2 uptake obtained for activated carbon (0.61 mmol g-1) which was prepared using coal tar pitch and furfural by steam activation.27 The demerits of the study is that CO2 uptake has been evaluated using volumetric apparatus under static flow condition which does not give the real picture for CO2 capture in a dynamic system.

Fig.

3

Breakthrough curves of CO2 (closed symbols) and N2 symbols) for a 12.5% CO2, the rest N2, on RF-700 at 30 °C and 50 °C


(open

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For carbon dioxide-nitrogen separation using RF-700, partial and total adsorbed amounts and x-y diagrams were obtained experimentally at different adsorption temperatures and gas phase concentrations. Pure component adsorption isotherms of CO2 and N2 were employed to predict binary system adsorption equilibria by using two different adsorption models along with application of IAST thermodynamic model in conjunction with those models. These predicted results are then compared with experimentally obtained data and are presented in Fig. 4.

Fig. 4 Total and partial adsorbed amounts of CO2 and N2 in binary system on RF-700 at (a) 30 °C, (b) 50 °C, (c) 75 °C, and (d) 100 °C (symbols represent experimental data and lines represent model predicted data) It was found that none of the employed models were able to predict CO2 adsorption isotherm in binary system accurately. DSL isotherm model predicted higher CO2 uptake than Sips equation at 30 °C while adsorption equilibria at rest of the adsorption temperatures was almost equally predicted by all the isotherm models along with their IAST forms. However,


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all the equations highly under predicted the uptake of CO2 on RF-700. Prediction of N2 uptake and hence total adsorbed amount also deviated considerably from experimental values at all adsorption temperatures. This can be attributed to difference in the adsorptive properties of adsorbates which is not being accounted by these isotherm equations.

Total adsorbed amount along with CO2 adsorbed amount increased with increase in CO2 composition in the binary mixture whereas amount of N2 continuously decreased, demonstrating competition for adsorption sites and preferential adsorption of CO2 over N2. Higher critical temperature of CO2 (31 °C) as compared to that of N2 (-147 °C) is the main reason for this behavior28,

29

because of intermolecular forces of attraction are directly

proportional to its critical temperature and it is higher for CO2, therefore CO2 gets more preference in competition for adsorption site. CO2 is expected to behave as a condensable vapor rather than a supercritical gas thereby becoming less volatile and getting adsorbed easily. Additionally, carbon dioxide exhibits higher polarizability than nitrogen (Table 4) that may enhance attractive forces with the adsorbent surface and a permanent quadrupole results in stronger interactions with carbon surface. Table 4 Physical properties of CO2 and N230 Properties

CO2

N2

Diameter (nm)

0.330

0.364

Polarizability (× 10 cm3)

29.1

17.4

-13.7

-4.9

0

0

Quadrupole moment (× 10 cm2) Dipole moment (Debye)


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Fig. 5 x-y diagrams for binary adsorption of CO2 and N2 on RF-700 at (a) 30 °C, (b) 50 °C, (c) 75 °C, and (d) 100 °C Experimental and model predicted molar fractions of CO2 in adsorbed phase vs. mole fraction of CO2 in gas phase at different adsorption temperatures for the CO2-N2 binary system were determined and shown in Fig. 5. CO2 mole fraction in adsorbed phase was found to increase with CO2 mole fraction in gas phase thereby indicating positive deviation from Raoult’s law at all adsorption temperatures. Total amount adsorbed was found to increase with molar fraction of CO2 in gas phase with non-symmetric x-y diagram thereby implying formation of non-ideal adsorptive mixture of CO2 and N2 at equilibrium. As the assumptions of IAST are not satisfied in this case, it results in large deviation from the experimental data. Moreover, Sips and DSL equations for binary system also deviated greatly from the experimental curves indicating the inability of these models to explain binary system adsorption equilibria of CO2 and N2 on nanostructured carbon RF-700. Table 5 reports the comparison between selectivity of CO2 over N2 obtained from experimental breakthrough curves and isotherm model predicted values. Experimental


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selectivity of CO2 over N2 was found to increase with gas phase molar fraction at all adsorption temperatures while all the isotherms predicted a decrease in the selectivity. IAST based DSL model predicted a small decrease in selectivity from 35.8 to 35.1 with increase in CO2 feed fraction from 0.05 to 0.125 at 30 °C whereas experimental values showed an increase from 48.9 to 83.1 under same conditions. Rise in temperature did not have a substantial effect on experimental values of selectivity for a given CO2 feed fraction but a decrease in the selectivity was predicted by all the isotherm models. Both Sips and DSL isotherm models along with their IAST based equations predicted lower selectivity values as compared to experimental values at all gas phase compositions and adsorption temperatures. This could be mainly due to under prediction of CO2 equilibria by these models. Hence it can be said that prediction of CO2/N2 selectivity is highly dependent on the accuracy of isotherm model. Table 5 Experimental and isotherm model predicted selectivity for CO2-N2 binary system on RF-700 CO2 mole Temp.

fraction in feed

Model predicted selectivity Experimental selectivity

Sips

DSL

IAST-Sips

IAST-DSL

model

model

model

model

0.050

48.9

28.9

35.7

40.3

35.8

0.075

51.6

26.3

34.8

39.1

35.4

0.100

58.5

24.6

34.0

38.5

35.2

0.125

83.1

23.3

33.4

38.1

35.1

0.050

45.8

24.4

28.9

30.5

28.0

0.075

50.1

22.7

28.7

29.6

27.8

0.100

49.5

21.5

28.5

29.1

27.7

0.125

57.3

20.6

28.3

28.8

27.7

0.050

46.9

19.1

22.7

22.2

21.0

0.075

56.2

18.2

22.9

21.8

21.0

0.100

55.5

17.6

23.2

21.6

20.9

30 °C

50 °C

75 °C


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0.125

61.8

17.2

23.4

21.5

20.9

0.050

48.9

14.5

16.9

15.9

15.7

0.075

58.2

14.2

17.2

15.7

15.7

0.100

76.6

14.0

17.5

15.7

15.8

0.125

76.7

13.9

17.8

15.6

15.8

100 °C

These isotherm models are based on the major assumption of equal adsorption capacities of two components which is practically not the case. Hence, it is difficult to predict adsorption equilibria of dissimilar adsorbates by these isotherm equations. Also as prediction by IAST involves extrapolation of pure component isotherms outside the experimental pressure range, isotherm of less strongly adsorbed component (i.e. N2 in this case) is extrapolated to a greater extent thereby introducing ambiguity in IAST predictions. Different polarities and quadrupole moment of CO2 and N2 results in large difference in interaction strength between different mixture components. Thus, both adsorbent heterogeneity and unlike adsorbates lead to nonconformity between IAST based values and experimental values. IAST has been found to accurately predict multicomponent adsorption equilibria but mainly at high pressure conditions. For example, Schell et al.24 reported that IAST using the pure component Sips parameters described competitive behavior of CO2/N2 mixtures on commercial activated carbon under high pressure conditions accurately. In another work, adsorption equilibria and selectivity of CO2 in binary system of CO2-N2, on mesoporous activated carbon, at 18 bar was explained accurately by IAST based dual-site Sips isotherm model.31 It should also be noted that adsorption equilibrium in these systems is typically evaluated by volumetric and gravimetric methods. But in the present work, predicted adsorption capacities do not match with the experimental data mainly due to different method (dynamic method) and adsorption conditions (atmospheric pressure and low CO2 mole fraction in gas phase) adopted for evaluation of experimental values.


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4. Thermodynamic study 4.1 Thermodynamic parameters In addition to equilibrium study, thermodynamic study was also carried out to characterize the adsorption behavior. Fig. 6 displays the calculated thermodynamic parameters i.e. ∆G, ∆H and ∆S×T for pure component adsorption of CO2 and N2 on RF-700 carbon at 30 °C and 50 °C by using Sips equation. All the thermodynamic parameters showed similar behavior at various adsorption temperatures. Negative values of ∆G for adsorption of CO2 and N2 on RF-700 over the complete experimental pressure range indicated the adsorption process to be spontaneous in nature. With rise in pressure, integral molar Gibbs free energy change of adsorption declined for both adsorbates at all temperatures suggesting lower work requirement during initial adsorption phase than later stages for packing of more molecules into cavities of carbon material. Value of ∆G was found to be -2.53±0.16 kJ mol-1 and -2.67±0.03 kJ mol-1 for CO2 and N2 respectively for the same coverage value of 0.18 mmol g-1 at 30 °C. This indicates that CO2 loading on RF-700 carbons has slightly lower work requirement as compared to N2 and higher pressure and hence higher chemical potential is required by N2 than CO2 to load on the prepared carbons.32, 33

Fig. 6 Thermodynamic functions (∆G, ∆H and ∆S×T) for CO2 (closed symbols) and N2 (open symbols) on RF-700 at (a) 30 °C, and (b) 50 °C Next, integral molar entropy change was evaluated and the function ∆S×T is presented in Fig. 6. At low pressure of 0.05 atm, ∆S×T for CO2 and N2 was -10.43±0.13 and -6.88±0.05 kJ mol-1 respectively at 30 °C while at higher pressure value (1 atm) the corresponding values were -13.44±0.04 and -7.53±0.01 kJ mol-1. Higher magnitude of this function for adsorption


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of CO2 than N2 indicated more ordered arrangement by CO2 on adsorbent surface. In other words, CO2 confined in the cavities of the adsorbent has less degree of freedom than N2 and this is on account of almost three folds quadrupole moment of CO2 than N2 and hence localized adsorption of CO2.12 Integral molar entropy change also showed negative values for both CO2 and N2 over the complete experimental temperature and pressure range thereby indicating formation of ordered configuration from random stage on adsorption with decrease in degree of freedom of adsorbate. Also, this thermodynamic parameter decreased progressively with increase in pressure for N2 adsorption. But in case of CO2 adsorption, there was decrease in entropy at 30 °C while at rest of the adsorption temperatures; it was almost constant over the complete pressure range.

Integral molar enthalpy change and function ∆S×T of adsorption of CO2 and N2 displayed similar trends and negative values of ∆H demonstrated exothermic nature of adsorption process 34. For CO2 adsorption at 30 °C, as pressure changed from 0.01 to 1.0 atm, ∆H varied from -9.97±0.19 to -17.88±0.04 kJ mol-1. However for N2, this value ranged from -7.43±0.10 to -10.27±0.02 kJ mol-1. There was gradual decrease in enthalpy values (more in case of CO2) i.e. higher heat release during adsorption process which was due to more effective packing of CO2 molecules. Same inference was obtained from trend of entropy change for CO2 adsorption. But at higher adsorption temperatures, ∆H showed almost constant values for pressure > 0.1 atm (ca. -23.5 kJ mol-1 for CO2 adsorption at 50 °C). Above evaluated parameters explain the pure CO2 and N2 adsorption on RF-700. However, assessment of these parameters for binary adsorption is unworkable due to lack of an accurate isotherm equation that could define binary adsorption equilibria.

Isosteric heat of adsorption for the adsorption of pure CO2 from equilibrium isotherms is calculated by using Clausius-Clapeyron equation (eq. 22 of supporting information) and is shown in Fig. 7. Average value of isosteric heat of adsorption (Qst) for CO2 on RF-700 was calculated to be 28.97±0.50 kJ mol-1 which is in agreement with the available value in literature for carbon adsorbents reported to be in the range of 11–30 kJ mol-1.35-38


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Fig. 7 Isosteric heat of adsorption with surface coverage on RF-700

5. CONCLUSIONS Pure component (CO2 and N2) adsorption isotherms on oxygen enriched nanostructured carbon adsorbent have been measured volumetrically at four different adsorption temperatures. Three different isotherm models namely Langmuir, Sips and Dualsite Langmuir isotherm models have been used for pure component adsorption equilibria study. Sips and DSL isotherm models provide best fit over the entire pressure range. Adsorption equilibria of binary component CO2 and N2 at four different adsorption temperatures (30-100 °C) and CO2 concentrations (5-12.5%) have been measured on RF-700 using a fixed bed column. For predicting binary component adsorption equilibria, extended Sips, extended dual-site Langmuir, IAST based Sips and IAST based DSL models have been used. Sips and DSL, IAST based Sips and IAST based DSL failed in predicting adsorption behavior of binary component. Neither, selectivity nor total adsorbed amount can be explained properly by these models because of high adsorption strength of one component as compared to the other and heterogeneous adsorbent surface. Overall, it can be concluded that binary component adsorption equilibria data are important for real system even they are time consuming. These data help in better selecting the appropriate model for simulation for gas phase adsorption systems. Molar Gibbs free energy change, entropy change, and enthalpy change for pure component signify a spontaneous, exothermic and feasible nature of adsorption process. Effective packing of CO2 molecules in the cavities of carbon adsorbents has been confirmed by higher amount of heat generation for CO2.


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ACKNOWLEDGEMENTS The author would like to greatly acknowledge financial support provided by Department of Science and Technology (DST) (scheme no. DST/IS-STAC/CO2-SR154/12(G).


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20. Tiwari, D.; Bhunia, H.; Bajpai, P. K., Synthesis of nitrogen enriched porous carbons from urea formaldehyde resin and their carbon dioxide adsorption capacity. J. CO2 Utilization 2017, 21, 302-313. 21. Goel, C.; Bhunia, H.; Bajpai, P. K., Resorcinol–formaldehyde based nanostructured carbons for CO2 adsorption: kinetics, isotherm and thermodynamic studies. RSC Adv. 2015, 5, (113), 93563-93578. 22. Do, D. D., Adsorption Analysis: Equilibria and Kinetics. Imperial College Press: 1998. 23. García, S.; Gil, M.; Martín, C.; Pis, J.; Rubiera, F.; Pevida, C., Breakthrough adsorption study of a commercial activated carbon for pre-combustion CO2 capture. Chemical Engineering J. 2011, 171, (2), 549-556. 24. Schell, J.; Casas, N.; Pini, R.; Mazzotti, M., Pure and binary adsorption of CO2, H2, and N2 on activated carbon. Adsorption 2012, 18, (1), 49-65. 25. García, S.; Pis, J. J.; Rubiera, F.; Pevida, C., Predicting Mixed-Gas Adsorption Equilibria on Activated Carbon for Precombustion CO2 Capture. Langmuir 2013, 29, (20), 60426052. 26. Caldwell, S. J.; Al-Duri, B.; Sun, N.; Sun, C.-g.; Liu, H.; Snape, C. E.; Li, K.; Wood, J., Carbon dioxide separation from nitrogen/hydrogen mixtures over activated carbon beads: adsorption isotherms and breakthrough studies. Energy Fuels 2015, 29, (6), 3796-3807. 27. Balsamo, M.; Budinova, T.; Erto, A.; Lancia, A.; Petrova, B.; Petrov, N.; Tsyntsarski, B., CO2 adsorption onto synthetic activated carbon: kinetic, thermodynamic and regeneration studies. Sep. Purif. Technol. 2013, 116, 214-221. 28. Rios, R. B.; Correia, L. S.; Bastos-Neto, M.; Torres, A. E. B.; Hatimondi, S. A.; Ribeiro, A. M.; Rodrigues, A. E.; Cavalcante, C. L.; de Azevedo, D. C., Evaluation of carbon dioxide–nitrogen separation through fixed bed measurements and simulations. Adsorption 2014, 20, 945-957. 29. Rios, R.; Stragliotto, F.; Peixoto, H.; Torres, A.; Bastos-Neto, M.; Azevedo, D.; Cavalcante Jr, C., Studies on the adsorption behavior of CO2-CH4 mixtures using activated carbon. Braz. J. Chem. Eng. 2013, 30, 939-951. 30. Wu, Y.-J.; Yang, Y.; Kong, X.-M.; Li, P.; Yu, J.-G.; Ribeiro, A. M.; Rodrigues, A. E., Adsorption of pure and binary CO2, CH4, and N2 gas components on activated carbon beads. J. Chem. Eng. Data 2015, 60, (9), 2684-2693. 31. Shao, X.; Feng, Z.; Xue, R.; Ma, C.; Wang, W.; Peng, X.; Cao, D., Adsorption of CO2, CH4, CO2/N2 and CO2/CH4 in novel activated carbon beads: Preparation, measurements and simulation. AIChE J. 2011, 57, (11), 3042-3051. 32. Yi, H.; Wang, Z.; Liu, H.; Tang, X.; Ma, D.; Zhao, S.; Zhang, B.; Gao, F.; Zuo, Y., Adsorption of SO2, NO, and CO2 on Activated Carbons: Equilibrium and Thermodynamics. J. Chem. Eng. Data 2014, 59, (5), 1556-1563. 33. Zhou, X.; Yi, H.; Tang, X.; Deng, H.; Liu, H., Thermodynamics for the adsorption of SO2, NO and CO2 from flue gas on activated carbon fiber. Chemical Engineering J. 2012, 200–202, 399-404. 34. Suresh, S.; Srivastava, V. C.; Mishra, I. M., Isotherm, thermodynamics, desorption, and disposal study for the adsorption of catechol and resorcinol onto granular activated carbon. J. Chem. Eng. Data 2011, 56, (4), 811-818. 35. Esteves, I. A. A. C.; Lopes, M. S. S.; Nunes, P. M. C.; Mota, J. P. B., Adsorption of natural gas and biogas components on activated carbon. Sep. Purif. Technol. 2008, 62, (2), 281-296. 36. Guo, B.; Chang, L.; Xie, K., Adsorption of carbon dioxide on activated carbon. J. Nat. Gas Chem. 2006, 15, (3), 223-229.


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37. Hsu, S.-C.; Lu, C.; Su, F.; Zeng, W.; Chen, W., Thermodynamics and regeneration studies of CO2 adsorption on multiwalled carbon nanotubes. Chem. Eng. Sci. 2010, 65, (4), 1354-1361. 38. Wahby, A.; Silvestre-Albero, J.; Sepúlveda-Escribano, A.; Rodríguez-Reinoso, F., CO2 adsorption on carbon molecular sieves. Microporous Mesoporous Mater. 2012, 164, (0), 280-287.


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Highlights

•

Adsorption isotherms of pure and binary components on oxygen-enriched nanostructured carbon (RF-700).

•

Sips and DSL adsorption isotherm models fitted well, indicating heterogeneous adsorbent surface.

•

Extended Sips, DSL equations and IAST indicating failed in predicting CO2 adsorbed amount.


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SUPPORTING INFORMATION Pure and binary gas adsorption equilibrium for CO2-N2 on oxygen enriched nanostructured carbon adsorbents Chitrakshi Goel, Deepak Tiwari, Haripada Bhunia,* Pramod K. Bajpai [email protected], [email protected], [email protected], [email protected] Department of Chemical Engineering, Thapar University, Patiala-147004, Punjab, India

*Correspondence to: Haripada Bhunia Email address: [email protected] Phone: +91-175-2393439; fax: +91-175-2393005


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1. Isotherm study 1.1 Pure component isotherms Equilibrium adsorption data of pure components on prepared carbon were correlated with Langmuir, Sips and dual-site Langmuir adsorption isotherm models.

1.1.1 Langmuir isotherm model Langmuir isotherm model assumes no interaction between the adsorbed molecules, monolayer adsorption on finite number of localized adsorption sites, with each site being identical and energetically equivalent and holding only one adsorbate molecule.1 The Langmuir isotherm model can be expressed as:

=

where

(1) 1 +

qe (mmol g-1) = equilibrium adsorption capacity, qm (mmol g-1) = maximum monolayer capacity, b (atm-1) = Langmuir parameter related to free energy of adsorption and measure of the affinity of the adsorbed molecules to the solid adsorbent surface, P (atm) = pressure.

Temperature dependence is explained by following Arrhenius type equation:

= (⁄) (2)

where

bo = adsorption affinity at reference temperature and B (J mol-1) = heat of adsorption.

1.1.2 Sips isotherm model Sips isotherm model is a three parameter adsorption isotherm model with parameters qm, b and c and can be written as:

⁄

= (3) 1 + ⁄

Here, c describes the system heterogeneity which could be due to adsorbent, adsorbate or their combination.2 This model is applied when complex pure and surface structures of the adsorbent causes the heterogeneity of the adsorption process because of both the adsorbent


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and the adsorbate materials instead of adsorbent material only thereby hardly satisfying the assumptions of Langmuir isotherm model.

1.1.3 Dual-site Langmuir model

= where

, , + (4) 1 + 1 +

, and , = saturation capacities of the adsorbate at sites 1 and 2 respectively

b1 and b2 = affinity parameters on sites 1 and 2 respectively.3

1.1.4 Error calculation

∑%& ,'( − ,(*+ ,- ,'( . ! (%) = # × 100 (5) /−1

where

,'( and ,(*+ = experimental and model predicted amounts of adsorbate adsorbed

respectively and

N = total number of data points.

1.2 Binary component isotherms Multicomponent adsorption equilibria prediction depends completely on the accuracy of pure component adsorption measurement and its reliable correlation with adsorption isotherm model. Binary system (CO2 and N2) adsorption equilibria have been predicted from pure component adsorption isotherm parameters by using two approaches. In the first approach, pure component adsorption isotherm equations have been extended empirically to binary isotherms while the second approach includes usage of ideal adsorbed solution theory (IAST) to derive binary system isotherms from pure component adsorption isotherms. Then, these predicted isotherms are compared with the measured binary system isotherms.

Extended Sips isotherm model for NC number of components Extended dual-site Langmuir isotherm model

,1 =

,1 =

,1 1 1

⁄2

5 1 + ∑36 3 3

4

, ,1 ,1 1

1+

⁄2

5 ∑4 36 ,3 3

(6)

+

, ,1 ,1 1

1+

5 ∑4 36 ,3 3


(7)

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• •

i = component for which isotherm is evaluated

,1 = equilibrium adsorption capacity of component i in the mixture

1.3 Ideal adsorbed solution theory (IAST) IAST is a well renowned technique, given by Myers and Prausnitz, for prediction of adsorption equilibria of a multi-component system using only pure component adsorption isotherms at the same temperature. IAST is a thermodynamic approach and is based on analogy of adsorption equilibria to Raoult’s law for vapor-liquid equilibrium:4-6

91 = 1 (: ∗ )

1 = =A D1 (9) ?@ 1 C

where

:1∗ and :1 = reduced spreading pressure and spreading pressure of component i in the

adsorbed phase respectively,

A = specific surface area of the adsorbent,

1∗ = pure component adsorption isotherm equation and

1 = standard state pressure of pure component i corresponding to spreading pressure of the mixture.

Standard state is defined as state at which reduced spreading pressure of the mixture (: ∗ ) is

the same as reduced spreading pressure for each component, i.e.

:1∗ = : ∗ F = 1, 2, 3 … /H (10)


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As there is no change in area on mixing for an ideal adsorbed mixture, total amount adsorbed ( ) can be calculated from pure component loadings at standard state by using following

equation:

45 1

Pure and Binary Gas Adsorption Equilibrium for CO2âN2 on Oxygen

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